1. Field of the Invention
The present invention generally relates to pin grid array sockets for connecting and disconnecting integrated circuit devices or packages to and from circuit boards, and more particularly to sockets where the insertion force to connect the device to the socket is zero and the device is maintained in the socket and the electrical connection is maintained under spring tension.
2. Description of the Prior Art
Low insertion force sockets have been well known in the art for securing electronic packages with a large number of leads in the form of pin contacts in a predetermined array onto a printed circuit board. Such sockets are used for making tests on the device and for inserting a device onto a printed circuit board. As the density of the circuits in a device increases, the number of connections is increased and the use of low or zero insertion force sockets increases. Further, instead of their use mainly for testing, devices may remain on circuit boards and the use of sockets which permit ready replacement of such devices is desirable.
Examples of the prior art sockets or connectors providing zero or low insertion force are found in U.S. Pat. Nos. 4,744,768; 4,343,524; 4,012,099; 3,763,459; and 3,569,905.
In some of these patents the contacts are formed to be in a normally closed position and this is afforded by the shape of the contact and the resilience of the material from which they are made. The contact force onto the leads of the device is determined by the individual contacts in this type socket. In this construction a cam is actuated to move a cam plate which serves to separate the legs of the contacts for insertion of the lead from the integrated circuit package between the legs of the contact.
In others, the contacts are normally open due to their design and are closed and held closed by the use of a cam which is rotated to move a cam plate such that the cam plate urges the legs of all the contacts to a closed position onto the leads of the devices by the force of a cam. In these devices the cam plate is made of plastic or polymeric material, as is the support for the cam, and in time there is a tendency for the polymeric material to cold flow and change the contact force on the lead of the package.
In U.S. Pat. No. 3,763,459 the socket described uses a leaf spring to urge the central section toward the open position, but as explained in Column 5, lines 28 through 33, the spring may be positioned to urge the central section toward a closed position. This patent further teaches using the force of a spring element under the bias of a crank to urge two cam plates in directions to afford the closing of the contacts onto the leads of a semiconductor device, see column 7, lines 51 through 57.
In U.S. Pat. No. 3,569,905 there is disclosed a normally open contact construction which is closed by a cam plate. In this device, the cam plate, rather than moving generally perpendicular to the lead to close the contact upon the lead, moves axially of the lead. The cam plate is urged in one direction to allow the opening of the contacts by a serpentine leaf spring or by a compression coil spring. The contacts are closed against the pressure of the springs by a fluid actuated chamber and to grasp the pin contact of an integrated circuit device. In this device as in the others, if the connection is to be maintained for any period of time there is going to be a loosening of the grip on the contact due to the escape of the fluid from the chamber, the cold flow of the polymeric material under the pressure of the springs, a weakening of the spring force of the individual contacts, or a combination thereof.
The present invention includes in the structure of the socket, a spring which is designed to prevent premature failure of a pin grid array socket in that it supplements the binding or connecting force and takes up the slack that can develop over time. The invention is described with respect to normally open socket contact constructions although the sockets described are placed in both categories. One embodiment is normally closed and can be opened by a pressure loading device during the insertion or removal of an integrated circuit package, and the other is normally open to receive the package and is closed onto the leads of the package by operation of a handle.